To get them super white add Tri Sodium Phosphate (TSP), the real kind not the non phosphate version.
Try not to do this often because phosphates are "not environmentally friendly".

M3

I use that stuff to clean my icky walls... to tell you how nasty the back half of the house is (we bought it very used and are remodeling and cleaning it up), I have to make a paste with the TSP and wash the walls at least 3 times before I can put Killz primer on it, and it takes 3-6 coats of primer to keep things (dirt, grease, blood) from bleeding though...